GCEP One-Year Exploratory Grant Fundamental Studies of Plasma Air Separation Investigators

This report describes our efforts of a one-year, exploratory study on the physics necessary to advance the development of a low power, small scale, air separation unit (ASU) based on non-equilibrium plasma discharges (PD-ASU). Our research focus is aimed at i) preliminarily design and selection of the test geometry of the air passage, plasma electrodes and discharge type, ii) constructing a prototype of the PD-ASU unit, and iii) simulating the unit using commercial software (COMSOL) to improve the separation performance. As for the design, a concentric electrode configuration, equipped with a copper/glass fiber-electrode dielectric barrier discharge (DBD) was selected and the corresponding prototype was constructed. Subsequent gas chromatographic analysis shows that the oxygen contents can be varied by approximately 1 % with this PD-ASU prototype. The simulation results show that the concentric PD-ASU design is effective in attracting negative ions (oxygen ions) toward a suction hole by concentrating the electric field while it is evenly distributed along linear electrodes with conventional DBD actuators. As an on-going project and future plans, a multistage PD-ASU (serial stacked version of our prototype) is being fabricated for improving the overall separation performance. Introduction Plasma discharge air separation is believed to have two potentially significant advantages over large scale cryogenic based ASUs, the state-of-the art process for separating air into oxygen and nitrogen. First, it is likely that the production of an enriched air stream will be more efficient than its cryogenic process counterpart, largely because the later requires significant heat removal to a thermal (warmer) reservoir thereby generating considerable entropy. Conversely, the process proposed here involves isothermal, but highly nonequilibrium kinetics, which can achieve the final state without thermodynamics limitations. The second potential advantage is one of practical significance – the likely portability of the PD-ASU can dramatically expand the use of pure oxygen combustion (to facilitate carbon dioxide sequestration) in vehicle transportation and in residential heating (where there are no practical separators), which is responsible for approximately 21 % of greenhouse gas emission [1]. We believe, however, that the PD-ASU can also replace large cryogenic based ASUs in some larger-scale operations. The development of a compact, and/or efficient PD-ASU using this selective plasma transport process can lead to a significant decrease, if not elimination, of greenhouse gas emission generated by automobiles and in residential and industrial furnaces. We envision a scenario where a compact air separator is installed upstream of the air intake in an automobile engine, natural gas boiler, or natural gas furnace (amongst other devices). Such a modified unit will also include a device that may make feasible, the capturing and storage of the CO2 emission, perhaps as a solid phase. The captured solid phase CO2 can be harvested from these energy conversion systems periodically and appropriately sequestrated. Such an energy conversion system can, in principle, be a zero-emissions energy conversion process. The ability to provide the technology to allow compact, portable, and efficient air separation can result in a new way to think about zero-emission vehicles and other processes that continue to burn hydrocarbon-based fossil fuels. Background Pure Oxygen Combustion At the center of increased greenhouse gas (particularly CO2) emissions is the extensive, if not exclusive, use of carbon-based fuels in contemporary energy systems. Three basic solutions to reducing CO2 emissions associated with energy conversion processes are a) increased efficiency in the utilization of carbon-based fuels, b) carbon-free resources, and c) nitrogen-free hydrocarbon combustion for facilitating CO2 sequestration in combustion product gases. This later process also reduces the production of nitrogen-related pollutants resulting from hydrocarbon combustion. One way to achieve nitrogen-free combustion is through so-called “oxy-combustion” [2], which uses an oxygen purified air stream (typically 95% O2). The products of pure oxygen combustion include CO2 and H2O, which are more readily sequestered. However, as mentioned above, state-of-the art pure oxygen combustion requires the energy intensive cryogenic process (the most widely used process) of air separation before combustion. The cost of this cryogenic separation technology is very high (some 0.24 eV per molecular oxygen), consuming approximately 28% of the entire power generated in a typical power plant operating at 40% efficiency [3]. Improvements in cryogenic air separation performance are likely to be incremental, in that the various thermomechanical processes associated with cryogenic air separation includes non-isentropic compression and expansion, evaporation, and heat transfer under large temperature gradients, leads to irreversible constraints and entropy generation not likely to be present in the nearisothermal process proposed below. Dielectric Barrier Discharge and Oxygen Forcing The typical dielectric barrier discharge (DBD) actuator that we have studied in our laboratory is a relatively novel, small scale plasma device, a schematic of which is shown in Fig. 1. Layered on top of a surface adjacent to a flow, it consists of an exposed thin conducting electrode and a second electrode that is buried below a dielectric material. The voltage (typically ~ 5 – 10 kV) between these two electrodes is driven at moderate frequencies (Hz to 100 kHz). Electrons released from the exposed electrode on the so-called “forward stroke” (that is, when it is negatively biased) migrate towards and can accumulate on the dielectric layer which covers the positively biased (at that time) buried electrode. The rise in surface charge will produce a countering field that will quickly lead to a condition of current self-termination. This self-termination usually occurs within a few tens of nanoseconds, resulting in nanosecond current bursts and a highly non-equilibrium discharge state. During the following “reverse stroke” (when the buried electrode reverts to be negative relative to the exposed electrode) the negative charge accumulated on the dielectric streams off of the surface and migrates towards to favored positive pole along the electric field. Research in our laboratory has discovered that during the DBD process in atmospheric pressure air flows, the dominant ionic charge carriers are negative ion of molecular oxygen (O2). During the early stages of the forward stroke, low energy electron attachment (releasing 0 – 0.2 eV/molecule) favorably forms O2, over the more endothermic electron impact ionization process (~15 eV/molecule) to form N2 (note that O2 and N2 are highly unstable). As a result, there is a strong ion drift towards the surface due to O2 migration, and hence an accumulation of O2 ions in the vicinity of the surface. These ions will displace both neutral O2 and N2, but since N2 is much more abundant in ambient air, the near surface region should be slightly enriched in oxygen. In essence, the plasma selectively transports oxygen across a diffusion layer. We describe in the next major section below, our experimental results that lead us to this discovery (until recently, no mention of negative oxygen ions was made in the air DBD literature). We also describe our plan to incorporate these findings into a discharge configuration that can effectively enrich air flows for air separation applications. Results Work during the year has focused on experimental and numerical investigations of the physics necessary to advance the development of a low power, small scale, air separation unit (ASU) based on non-equilibrium plasma discharges. During the first half, we have tested a few prototypes of the preliminary PD-ASU design and conducted numerical simulations suggesting an optimized ASU design concept. The optimization methodology was supported by an experimental result depicting that the plasma can selectively apply a body force on oxygen in air, and a simulation result providing a novel electrode design concentrating the electric field on a point. In the latter part of this study, various ASU cells were manufactured based on the optimized design concept and tested experimentally. Gas chromatography (GC) is used to estimate the performance of the unit by measuring the gas composition of the outlet gas. Also, a multi-stage ASU built in a pressure variable chamber is ready to operate for future research. plasma Figure 1. Schematic of a typical dielectric barrier